1. Technical Field
The present invention relates to retrieval of previously stored data, including data recorded in magnetic media and data stored in magnetic bubble memories, by a magnetroresistive (MR) read head or sensor. The method and apparatus of the present invention involve detection of the instantaneous value of the relative change, .DELTA.R.sub.h /R.sub.h, of the MR head sensor resistance, R.sub.h, in two different biasing configurations.
2. BACKGROUND ART
An MR head is an active or parametric transducer which requires an electrical current through its resistive sensor stripe to be active. The current serves as a sensing current for converting the stripe resistance variations produced by the field, Hy, emanating from the medium into voltage variations across the stripe.
The higher the current applied to the head, the larger the readout voltage. The magnitude of the current, however, must be limited to avoid overheating the sensor stripe, and to avoid electromigration of the stripe material. This current should be provided from a low noise source to minimize noise injected into the read channel electronics.
Most existing MR head preamplifiers bias the sensor stripe of the head at a constant bias current, I.sub.b, and detect the voltage variation .DELTA.V, (hereafter also referred to as signal voltage, V.sub.s), developed at the head terminals. Thus, EQU .DELTA.V=V.sub.s =I.sub.b .DELTA.R.sub.h, (A)
where .DELTA.R.sub.h is the absolute change in the head resistance, R.sub.h, owing to the magnetic input signal, H.sub.y, from the media being read.
Stripe height, the dimension of the sensing stripe perpendicular to the media will vary from device to device because of variations in the lapping process. Stripe height also changes as much as 50% over the life of an MR head in contact recording applications. Since both R.sub.h and .DELTA.R.sub.h are proportional to stripe height, .DELTA.R.sub.h /R.sub.h is independent of stripe height. It can also be shown the .DELTA.R.sub.h /R.sub.h substantially corrects for stripe thickness variations, and fully corrects for variations in stripe length. Thus, prior art preamplifiers which detect V.sub.s of equation (A) provide different sensitivity if switched from one head to another in a multihead storage device or, in contact recording devices, as the stripe height is worn away. Similarly, different sensitivities would be encountered when switching from one sensor to another in bubble memory systems.
R.sub.h typically exhibits a temperature coefficient of about 0.3 to 0.5% per degree C. Thus, as temperature varies, R.sub.h varies according to its particular temperature coefficient. The corresponding variation in .DELTA.R.sub.h causes low frequency noise modulation of the output signal, V.sub.s. Again, since .DELTA.R.sub.h and R.sub.h are both affected by temperature in the same way, .DELTA.R.sub.h /R.sub.h is substantially independent of temperature variations.
In other prior art configurations, U.S. Pat. No. 3,814,863 proposes biasing an MR head using resistors having large values with respect to the value of R.sub.h, and an AC coupled differential voltage amplifier. U.S. Pat. No. 4,040,113 describes a current source used for biasing a center-tapped MR element and an AC coupled differential voltage amplifier for detecting the output signal produced by the head. Finally, U.S. Pat. No. 4,191,977 describes a method of biasing a center-tapped MR head using two inductors in series with the head and a voltage source with a DC coupled differential voltage amplifier. Thus, the prior art teaches biasing the MR head with a constant bias current and detecting the signal produced across the head with a differential voltage amplifier. The detected signal, which is proportional to .DELTA.R.sub.h, is sensitive to production tolerances, contact recording wear, and temperature variations.